1. Field of the Invention
The present invention relates to a noncontact IC card reader/writer, particularly relates to a noncontact IC card reader/writer the efficiency of which is improved and the receive sensitivity of which is also improved by using an efficient class E amplifier for an amplifier and in addition, by branching send/receive at the end of an antenna in a matching circuit.
2. Description of the Related Art
Heretofore, a reading/writing system using an IC card has been generally called a noncontact IC card system and is being practically utilized for an entrance/outgoing management system utilizing a frequency band of 13.56 MHz for example, a physical distribution system, an air cargo management system and others.
This system is provided with a noncontact IC card 101 having an IC chip 103 and an antenna coil 102 on one card made of resin and a reader/writer 105 that communicates with the noncontact IC card 101 as shown in FIG. 3, and the reader/writer 105 is provided with a loop antenna 104. FIG. 3 is a conceptual drawing showing a conventional type noncontact IC card system. Electric power and send data are constantly or intermittently transmitted via the loop antenna 104 and receive data from the noncontact IC card 101 located in a range in which the electric power and the send data can be received is acquired.
Further, a reader/writer of a noncontact IC card system disclosed in JP-A-2002-7976 is shown in FIG. 4. FIG. 4 are block diagrams showing parts related to the connection of the reader/writer of the conventional type noncontact IC card system and a noncontact IC card. FIG. 4A shows a case that the loop antenna 104 and a capacitor form a parallel resonance circuit and FIG. 4B shows a case that the loop antenna 104 and a capacitor form a series resonance circuit. Referring to FIGS. 3 and 4, the operation of the noncontact IC card system in a conventional type example 1 will be described below.
First, in case send data is transmitted, a carrier wave from a prior oscillator is input to a modulator and is modulated based upon send data (not shown). The modulated wave is amplified in a power amplifier 106 shown in FIG. 4 and is transmitted from the loop antenna 104 via a matching circuit 107.
In case only electric power is transmitted, a carrier wave from the prior oscillator is transmitted as it is without modulation. The transmission from the reader/writer to the noncontact IC card 101 is made because a magnetic flux generated by the loop antenna 104 is interlinked with the antenna coil 102 of the noncontact IC card 101 and induced power is excited by electromagnetic coupling. On the side of the noncontact IC card 101, voltage induced by the antenna coil 102 is rectified in a rectifying circuit (not shown) in the IC chip 103 and is used for a power source of each circuit in the noncontact IC card 101. Besides, the same induced voltage is conducted to a demodulator circuit (not shown) and data from the reader/writer 105 is demodulated there.
Next, when data is transmitted from the noncontact IC card 101 to the reader/writer 105, the reader/writer 105 transmits an unmodulated carrier wave and supplies only electric power to the noncontact IC card 101. In a modulator circuit (not shown) composed of a load resistor (not shown) connected to the antenna coil 102 for example and a switch (not shown) on the side of the noncontact IC card 101, the switch is turned on or off according to a bit “1” or “0” of data read from a memory (not shown) in the IC chip 103. When the switch is turned on or off as described above, a load Z onto the antenna coil 102 varies, the variation is conducted to the loop antenna 104 on the side of the reader/writer 105 by electromagnetic induction, impedance on the side of the loop antenna 104 varies, and voltage/current, that is, impedance at a point A shown in FIG. 4A varies according to send data from the noncontact IC card 101. As a result, the amplitude of a high-frequency signal varies. That is, amplitude modulation is applied to the high-frequency signal depending upon data in the noncontact IC card 101. The modulated high-frequency signal is demodulated in a demodulator circuit 108 and receive data is acquired.
Besides, JP-A-10-256957 is known as an example that a class E amplifier is used for the readers/writers of these noncontact IC card systems and referring to FIG. 5, a conventional type example 2 will be described below. FIG. 5 is a block diagram showing a reader/writer of a conventional type noncontact IC card system. In FIG. 5, a part of the reader/writer disclosed in the patent document 2 is integrated in a simple block.
As shown in FIG. 5, a reference number 115 denotes an antenna, 116 denotes a class E amplifier, 117 denotes an impedance matching network, 118 denotes a capacitor, 119 denotes a filter circuit, 120 denotes CPU, Tr11 denotes a transistor, L11 denotes a coil, D11 denotes a diode, C12 denotes a second capacitor, L12 denotes a second coil, C13 denotes a third capacitor, C14 denotes a fourth capacitor, the class E amplifier 116 is connected to the antenna 112, CPU 120 and the impedance matching network 117, the other end of the impedance matching network 117 is connected to a power source, further, the class E amplifier 116 and the impedance matching network are connected to the filter circuit 119 via the coupling capacitor 118, and the other end of the filter circuit 119 is connected to CPU 120.
The class E amplifier 116 will be described in detail below. As shown in FIG. 5, the class E amplifier 116 uses the transistor Tr11 for a switching device, a base functions as an input terminal, an emitter is grounded, a collector is connected to the coil L11 the other end of which is connected to the power source and one end of the second coil L12, further, the diode D11 and the second capacitor C12 are connected in parallel between the collector and the ground, the other end of the second coil L12 is connected to the third capacitor C13, further, the other end of the third capacitor C13 is connected to the fourth capacitor C14 one end of which is grounded, and the other end of the third capacitor C13 functions as an output terminal.
Next, the operation of the reader/writer will be described. When the noncontact IC card is inserted into the reader/writer, a detecting circuit outside the drawing detects the IC card and sends a transmission start signal to CPU 120. CPU 120 that receives the transmission start signal turns on the power source of the class E amplifier 116 and sends a modulated data signal to the class E amplifier 116. In the class E amplifier 116, the modulated data signal is amplified, is shaped into a sine wave and is transmitted from the antennal. The send data signal output from CPU 120 is a digital signal, when the signal is at a high level, the transistor Tr11 is turned off, and when the signal is at a low level, the transistor is turned on. As described above, the transistor Tr11 executes switching operation, however, the leading edge and the trailing edge of the switching operation are temporally depressed at the collector of the transistor Tr11 by a circuit called a loading network composed of the coil L11, the second capacitor C12, the second coil L12 and the third capacitor C13, and is output from the collector in a state close to not a rectangular wave but a sine wave. According to this amplifying system called switching amplification, amplifying operation can be efficiently executed. In the case of the conventional type example 2, when the noncontact IC card is inserted, the transistor Tr11 is adjusted so that the operation of a class E is executed.
The noncontact IC card and the antenna of the noncontact IC card reader/writer can be regarded as a transformer when they communicate, and normally in such a case, as the transformer is often loosely coupled, reactance caused in a loose coupling state is negated by the fourth capacitor and stable operation is assured.
The noncontact IC card that receives a send signal responds with load fluctuation as in the conventional type example 1 and after an undesired wave is removed in the filter circuit 119 via the loading network of the class E amplifier in the case of the conventional type example 2, the load fluctuation is demodulated in CPU 120. In this conventional type example 2, the low-power consumption reader/writer of the noncontact IC card system is realized by making the amplifier operate at the class E only when the noncontact IC card approaches and a signal from it can be received as described above.
However, in case the reader/writer having large output of the noncontact IC card system is realized by the above-mentioned conventional type configuration, some problems occur.
First, in case no noncontact IC card exists around, the amplifier is not stably operated and the efficiency is deteriorated. Therefore, an insertion detector or a cooler is required and it is difficult to realize the small-sized and low-priced power transmitter described as the advantage of the patent document 2 described as the conventional type example 2. Second, as the loading network used for a filter for the output of the power amplifier has low-order filter circuit configuration, capacity to remove a high frequency is small and it is very difficult to inhibit extraneous emission. Third, not only the sensitivity is deteriorated because the potential of a receive signal is divided in a circuit including an impedance component but the efficiency of the amplifier is deteriorated. The transistor used for a switching device in the patent document 2 has various problems such as the efficiency of the amplifier is deteriorated because the ON-state resistance of the transistor is large.